Many astrophysical and cosmological subjects could be studied with their special GW signals. In Fig. REF , some important GW sources are shown, together with the sensitivity of several projected GW detectors. These detectors are divided into three categories: ground-based detectors [1]}, [2]}, [3]}, [4]}, space-based detectors [5]}, [6]}, [7]}, [8]}, and Pulsar timing arrays [9]}, [10]}, [11]}, corresponding to the GWs of high frequency, low frequency, and very low frequency, respectively. The GW sources could be the binaries (including BH binaries, neutron star binaries, and white-dwarf binaries), the rotating neutron stars, the stochastic background of supermassive binaries, supernovae, inflation, and so on.
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